CN111330631B - Preparation method of modified molecular sieve loaded Pd catalyst and application of modified molecular sieve loaded Pd catalyst in synthesis of dimethyl carbonate by gas phase method - Google Patents

Abstract

The invention belongs to the technical field of catalyst preparation, and in particular relates to a preparation method of a modified molecular sieve supported Pd catalyst and its application in the synthesis of dimethyl carbonate by a gas phase method. The carrier of the catalyst is modified FAU or EMT molecular sieve, the main active components are zero-valent palladium (Pd) and valence Pd, the mass fraction of Pd in the catalyst is 0.1%-2.5%, and the average particle size of Pd particles The size is 0.2 to 15 nanometers. The catalyst is used in the reaction of carbon monoxide and methyl nitrite low-pressure gas phase synthesis of dimethyl carbonate, solves the problems of equipment corrosion and deactivation caused by traditional chlorine-containing catalysts, and is a high stability, high selectivity, A high-performance catalyst with high conversion rate, anti-sintering, chlorine-free, and adjustable Pd nanoparticle size, the catalyst has a simple preparation method, low equipment requirements, and low production cost, is suitable for large-scale production, and has certain industrial application prospects .

Description

A kind of preparation method of modified molecular sieve supported Pd catalyst and carbon synthesis in gas phase method Application of Dimethyl Acid

technical field

The invention belongs to the technical field of catalyst preparation, and in particular relates to a preparation method of a modified molecular sieve supported Pd catalyst and its application in the synthesis of dimethyl carbonate by a gas phase method.

Background technique

Dimethyl carbonate (DMC) is an important environmentally friendly chemical raw material. It has been listed as a non-toxic chemical as early as 1992 and can be used in the production of polycarbonate, medicine, pesticides and other products, replacing phosgene and halogenated chemicals. Methane and dimethyl sulfate are used as carbonylation and methylation reagents as solvents for lithium battery electrolytes and paint coatings industry (accounting for more than 50% of my country's total DMC consumption), and are expected to replace toxic methyl tert-butyl ether ( MTBE) as gasoline and diesel additive. According to statistics, the market demand for DMC in my country has grown exponentially since 2007, and the demand for DMC in 2019 has reached 810,000 tons per year. It can be seen that DMC has a huge potential market and broad application prospects.

The methods for synthesizing dimethyl carbonate can be mainly divided into: phosgene method, oxidative carbonylation method, nitrite carbonylation method, transesterification method, methanol/carbon dioxide one-step synthesis method and urea alcoholysis method. The phosgene method is being phased out due to the disadvantages of high toxicity of raw materials, poor safety and serious environmental pollution. Although the transesterification method has the conditions of simple operation and mild reaction conditions, the separation and purification of the target product is relatively difficult and the cost is high. ; The uric acid alcoholysis method also encounters the difficulty of subsequent separation due to the use of homogeneous catalysts; although the one-step synthesis method of methanol and carbon dioxide is simple, the catalyst activity is low. The methanol conversion rate of the one-step synthesis method of methanol and carbon dioxide reported in Chinese patent CN110479287A is only 11.2%, the selectivity of dimethyl carbonate is 75.6%, and the yield is only 8.5%.

Relatively speaking, the low-pressure gas phase synthesis of dimethyl carbonate from carbon monoxide and methyl nitrite has attracted extensive attention due to the advantages of non-polluting production process, environmental friendliness, and no subsequent separation. The catalysts for the low-pressure gas phase synthesis of dimethyl carbonate from carbon monoxide and methyl nitrite mainly include chlorine-containing Pd-Cu/oxide and chlorine-free Pd/molecular sieve catalyst systems. Chlorine-containing catalysts require continuous addition of chlorine (such as 100ppm HCl) to the raw materials, which can cause serious corrosion of equipment and low purity of DMC products. For example, US Pat. No. 5,688,984 discloses a spinel-supported catalyst for the synthesis of dimethyl carbonate. It is necessary to add a chlorine-replenishing agent hydrogen chloride or methyl chloroformate in the raw material. The chlorine-replenishing agent hydrogen chloride can cause equipment corrosion, and Methyl chloroformate is very expensive. Therefore, the development of chlorine-free catalysts for the synthesis of dimethyl carbonate is the future development trend. Chinese Patent Application Publication No. CN106423289A publicly reported a catalyst for synthesizing dimethyl carbonate and a preparation method thereof, using copper and potassium as the auxiliary agent of the molecular sieve-loaded Pd catalyst, the space-time yield of this catalyst was 690g/(Lh), but The selectivity of dimethyl carbonate based on methyl nitrite is only 45% to 51%, which shows that the selectivity of the catalyst is low. A molecular sieve-supported Pd catalyst was prepared by Yamamoto of Japan's Ube Company. The selectivity of dimethyl carbonate based on methyl nitrite was 75%. After 150 hours of operation, the CO conversion rate even decreased to 75% of the initial conversion rate. It can be seen that the company The prepared molecular sieve supported Pd catalyst has poor stability (Catalysis and characterization of Pd/NaY for dimethyl carbonate synthesis from methyl nitrite and CO, Yamamoto et al., J.Chem.Soc.Faraday Trans., 1997, Vol. 93, No. 3721 pages). To sum up, the introduction of chlorine in chlorine-containing catalysts will lead to serious corrosion of equipment and low purity of DMC products. However, the conversion rate and selectivity of chlorine-free Pd/zeolite catalysts are low, and the stability is poor. There are still problems. Great room for improvement.

In the process of catalyst preparation, due to imperfect preparation methods or unsatisfactory distribution of noble metal catalysts, noble metal catalysts will have larger metal particles. For example, when traditional palladium-based catalysts are prepared by excess or equal amount of impregnation method, this impregnation method is used to prepare The distribution of Pd particles in the palladium-based catalyst is often uneven, and larger Pd particles will be produced, resulting in a catalyst with a lower metal dispersion, thereby increasing the production cost of the Pd catalyst. Peng et al. found that the Pd/ZnO catalyst prepared by the impregnation method had obvious sintering behavior during the catalytic reaction, and the catalyst had a lower Pd dispersion (Enhanced Stability of Pd/ZnO Catalyst for CO Oxidative Coupling to Dimethyl Oxalate: Effect of Mg). , Peng et al., ACS Catalysis, 2015, Vol. 7, p. 4410). Descorme et al. found that the Pd/ZSM-5 catalyst prepared by the liquid-phase ion exchange method was gradually deactivated during the catalytic reaction, mainly because the Pd particles gradually migrated to the edge of the molecular sieve, and the Pd particles even increased to 100 nanometers (Palladium-exchanged MFI-type zeolites in the catalytic reduction of nitrogen monoxide by methane. Influence of the Si/Al ratio on the activity and the hydrothermalstability, Descorme et al., Appl. Catal. B, 1997, Vol. 13, p. 185). It can be seen that how to control the dispersion of noble metal Pd catalyst is the most important factor in the preparation process of noble metal catalyst.

SUMMARY OF THE INVENTION

In view of the above problems existing in the prior art, the purpose of the present invention is to provide a modified molecular sieve supported Pd catalyst with high stability, high selectivity, high conversion rate, anti-sintering, no chlorine, and adjustable Pd nanoparticle size. The preparation method and application, in particular, provide a high-performance catalyst for the reaction of carbon monoxide and methyl nitrite to synthesize dimethyl carbonate by a low-pressure gas phase method.

In order to achieve the above purpose of the invention, the present invention provides a preparation method of a modified molecular sieve supported Pd catalyst and an application in the synthesis of dimethyl carbonate by a gas phase method, wherein the method comprises the following steps:

1) at a temperature of 20-95 °C, treating the molecular sieve carrier with a modified solution, filtering, washing, drying, and calcining at a temperature of 150-550 °C for 2 hours to obtain a modified molecular sieve;

2) adding the modified molecular sieve of step 1) into the aqueous solution and stirring to form a suspension, dissolving the palladium precursor with a dilute aqueous ammonia solution of 0.2-5.0 mmol/L to obtain a mixed solution, adding the mixed solution to the suspension, and controlling The mass ratio of palladium element and modified molecular sieve is 0.001～0.025:1. According to the pH value of the mixed solution, use inorganic acid or inorganic base to adjust the pH of the solution to 5～10, and stir the reaction at 5～95℃ for 0.5～48 After filtration, washing, drying at 5-95°C for 1-48 hours, and then roasting in a muffle furnace to obtain Pd-loaded molecular sieves;

3) Put the Pd-loaded molecular sieve obtained in step 2) into a mold for molding, and change the number of molecular sieve defect sites through catalyst molding conditions, thereby changing the size of the Pd particles, and finally obtaining a modified molecular sieve-loaded Pd catalyst.

As a preferred solution, the molecular sieve in the step 1) is one or a combination of FAU molecular sieve and EMT molecular sieve.

As a preferred version, the modified solution in the described step 1) is sodium hydroxide, potassium hydroxide, sodium chloride, sodium carbonate, sodium bicarbonate, sodium nitrate, sodium acetate, potassium chloride, potassium carbonate, potassium bicarbonate , potassium nitrate, potassium acetate, acetic acid, oxalic acid, hydrochloric acid, nitric acid, succinic acid, citric acid, ethylenediaminetetraacetic acid, hydrofluoric acid, ammonium fluoride, one or more combinations, the concentration is 0.01 ~ 0.5mol /L, the treatment time is 0.5 to 24 hours.

As a preferred version, in the described step 2), the palladium precursor is a kind of palladium nitrate, palladium acetate, palladium chloride, ammonium chloropalladate, potassium chloropalladate, tetraammine palladium chloride, tetraammine palladium nitrate or a combination of several; the inorganic acid includes one or more combinations of hydrochloric acid, nitric acid, acetic acid, and oxalic acid; the inorganic base includes ammonia, sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate , potassium carbonate, potassium bicarbonate one or more combinations.

As a preferred solution, in the step 2), the heating rate of the muffle furnace is 0.2-2.0°C/min, the roasting temperature is 110-250°C, and the roasting time is 0.5-4 hours.

As a preferred solution, in step 3), the catalyst molding temperature is 10-150° C., and the molding pressure is 10-1000 MPa.

As a preferred solution, the active components of the modified molecular sieve supported Pd catalyst obtained in step 3) are zero-valent Pd and valence Pd, the carrier is modified molecular sieve, and the mass fraction of Pd in the catalyst is 0.1% to 2.5%, The average particle size of the Pd particles is 0.2 to 15 nanometers.

As a preferred solution, in the modified molecular sieve-supported Pd catalyst obtained in step 3), the molecular sieve has an average particle size of 0.1-4 microns, a pore volume of 0.21-0.37 cm ³ /g, and a specific surface area of 800-950 m ² /g.

Compared with the prior art, the preparation method of a modified molecular sieve-supported Pd catalyst of the present invention and the application in the gas-phase synthesis of dimethyl carbonate, (1) the present invention has discovered modified EMT and FAU molecular sieve-supported Pd catalyst For the synthesis of dimethyl carbonate, it has good stability, selectivity and conversion rate, which is related to the special pore structure and cation distribution of EMT and FAU molecular sieves; (2) The molecular sieve is pretreated with a modified solution, which can open the pores of the molecular sieve. , so that graded pores are formed inside the molecular sieve, which will limit the migration of Pd particles to the edge of the molecular sieve and prevent the sintering of Pd particles; (3) The heating rate and calcination temperature during the calcination process of the catalyst are an important factor to control the size distribution of Pd particles , when the heating rate is too fast and the calcination temperature is too high, Pd is easy to sinter, forming larger Pd particles without catalytic activity. The pores of the molecular sieve reduce the catalytic activity of the catalyst; (4) The molding conditions of the catalyst are a key factor in adjusting the size of the Pd particles. When the molding temperature and molding pressure are too low, the defect sites inside the molecular sieve are few, and the Pd easily migrates to the molecular sieve. At the edge, larger Pd particles are formed; when the molding temperature and molding pressure are too high, there will be too many defects inside the molecular sieve, and the molecular sieve will be severely broken, and Pd will also easily form larger Pd particles in the severely broken area; when the molding temperature When the forming pressure is within a reasonable range, the defect sites inside the molecular sieve are moderate, and the Pd particle size is small at this time, which can effectively prevent Pd from migrating to the edge of the molecular sieve particle. It can be seen that the preparation method of a modified molecular sieve-supported Pd catalyst of the present invention and its application in the synthesis of dimethyl carbonate by gas phase method can effectively prevent the sintering of Pd, improve the dispersion of Pd, and the prepared modified Molecular sieve-supported Pd catalyst is a catalyst with high stability, high selectivity, high conversion, anti-sintering, chlorine-free, and adjustable Pd nanoparticle size, especially for the low-pressure gas phase synthesis of dimethyl carbonate from carbon monoxide and methyl nitrite. Ester reactions provide a high performance catalyst.

The beneficial effects of the present invention are as follows: the present invention finds that modified EMT and FAU molecular sieve are excellent catalyst carriers, and the modified solution is used to form graded pores inside the molecular sieve, and the size distribution of the Pd particles is controlled by the heating rate and the roasting temperature in the roasting process, The size of the Pd particles is adjusted by the molding conditions of the catalyst. The average particle size of the Pd particles is 0.2-15 nanometers. It shows excellent catalytic performance in the low-pressure gas phase synthesis of carbon monoxide and methyl nitrite. The conversion rate is greater than 85%, the selectivity of dimethyl carbonate based on methyl nitrite is greater than 83%, and it can operate stably for more than 300 hours.

Description of drawings

Fig. 1 is the SEM image of the modified molecular sieve supported Pd catalyst prepared in Example 1;

2 is the XRD pattern of the modified molecular sieve supported Pd catalyst prepared in Example 1;

3 is a TEM image of the modified molecular sieve supported Pd catalyst prepared in Example 1;

4 is the SEM image of the modified molecular sieve supported Pd catalyst prepared in Example 2;

5 is the XRD pattern of the modified molecular sieve supported Pd catalyst prepared in Example 2;

6 is a TEM image of the modified molecular sieve supported Pd catalyst prepared in Example 2;

7 is the SEM image of the modified molecular sieve supported Pd catalyst prepared in Example 3;

8 is the XRD pattern of the modified molecular sieve supported Pd catalyst prepared in Example 3;

9 is a TEM image of the modified molecular sieve supported Pd catalyst prepared in Example 3;

10 is the SEM image of the modified molecular sieve supported Pd catalyst prepared in Example 4;

Fig. 11 is the nitrogen adsorption desorption of the modified molecular sieve supported Pd catalyst prepared in Example 4;

12 is the XRD pattern of the modified molecular sieve supported Pd catalyst prepared in Example 4;

Figure 13 is a TEM image of the modified molecular sieve supported Pd catalyst prepared in Example 4;

Figure 14 is the XRD pattern of the modified molecular sieve supported Pd catalyst prepared in Comparative Example 1;

Figure 15 is a TEM image of the modified molecular sieve supported Pd catalyst prepared in Comparative Example 1;

FIG. 16 is the stability test chart of the modified molecular sieve-supported Pd catalyst prepared in Example 2. FIG.

Detailed ways

In order to facilitate the understanding of the content of the present invention, the present invention now cites the following examples. The examples are only to help the understanding of the present invention, and should not be regarded as a specific limitation of the present invention. Because the present invention can also be described and explained by other solutions that do not depart from the technical features of the present invention, all changes within the scope of the present invention or within the scope of the equivalent invention should belong to the protection scope of the present invention.

Below in conjunction with embodiment, comparative example and application example, the present invention is further elaborated.

Example 1

1) Utilize the mixed solution containing 0.02mol/L sodium bicarbonate and 0.03mol/L sodium chloride to pretreat FAU molecular sieve at 20°C for 0.6 hours, wash, dry at 80°C for 4 hours, and roast at 155°C for 2 hours , to obtain a modified molecular sieve;

2) adding the modified molecular sieve in step 1) into deionized water to form a suspension, dissolving palladium chloride in a 0.25mmol/L dilute aqueous ammonia solution to obtain a mixed solution, adding the mixed solution dropwise to the suspension to control the palladium element The mass ratio with the modified molecular sieve was 0.002:1, the pH was adjusted to 5.5 with an aqueous hydrochloric acid solution, and the reaction was stirred at 6 °C for 0.6 hours. The heating rate of 0.25°C/min was increased to 115°C, and kept in a muffle furnace for 0.6 hours to obtain a Pd-loaded molecular sieve;

3) Put the Pd-loaded molecular sieve obtained in step 2) into a mold, shape the catalyst at 10° C. and 20 MPa, and finally obtain a modified molecular sieve-loaded Pd catalyst.

Figure 1 is a SEM image of the modified molecular sieve supported Pd catalyst prepared in this example, and it can be seen that the average particle size of the molecular sieve is 0.3 microns. The catalyst had a pore volume of 0.22 cm ³ /g and a specific surface area of 830 m ² /g as measured by nitrogen adsorption and desorption. Fig. 2 is the XRD pattern of the modified molecular sieve-supported Pd catalyst prepared in the present embodiment. It can be seen from the figure that there is no diffraction peak of metal palladium, indicating that Pd has a high degree of dispersion. The mass content of Pd was 0.2% as measured by inductively coupled plasma emission spectrometer. FIG. 3 is a TEM image of the modified molecular sieve-supported Pd catalyst prepared in this example after 12 hours of catalytic reaction, and it can be seen that the average particle size of the Pd particles is 6.3 nanometers.

Example 2

1) Using a mixed solution containing 0.20 mol/L acetic acid and 0.10 mol/L oxalic acid to pretreat FAU molecular sieves at 40°C for 6.0 hours, washing, drying at 80°C for 4 hours, and calcining at 300°C for 2 hours to obtain modified molecular sieve;

2) adding step 1) modified molecular sieve into deionized water to form a suspension, dissolving palladium nitrate and palladium acetate in a dilute aqueous ammonia solution of 2.4 mmol/L to obtain a mixed solution, adding the mixed solution dropwise to the suspension, and controlling The mass ratio of palladium element and modified molecular sieve was 0.011:1, the pH was adjusted to 7.5 with nitric acid and acetic acid aqueous solution, the reaction was stirred at 35 °C for 24 hours, filtered and washed, dried at 45 °C for 24 hours, and then used The muffle furnace was raised to 180 °C at a heating rate of 0.5 °C/min, and kept in the muffle furnace for 2 hours to obtain the Pd-loaded molecular sieve;

3) Put the Pd-loaded molecular sieve obtained in step 2) into a mold, shape the catalyst at 40° C. and 350 MPa, and finally obtain a modified molecular sieve-loaded Pd catalyst.

Figure 4 is a SEM image of the modified molecular sieve supported Pd catalyst prepared in this example, and it can be seen that the average particle size of the molecular sieve is 2.8 microns. The catalyst had a pore volume of 0.30 cm ³ /g and a specific surface area of 880 m ² /g as measured by nitrogen adsorption and desorption. Fig. 5 is the XRD pattern of the modified molecular sieve-supported Pd catalyst prepared in the present example. It can be seen from the figure that there is no diffraction peak of metal palladium, indicating that Pd has a high degree of dispersion. The mass content of Pd was 1.1% as measured by inductively coupled plasma emission spectrometer. Figure 6 is a TEM image of the modified molecular sieve supported Pd catalyst prepared in this example after 12 hours of catalytic reaction. From the TEM image, an obvious molecular sieve lattice can be seen, and the Pd particles are evenly embedded in the molecular sieve lattice. The average particle size of the particles was 0.3 nm.

Example 3

1) using 0.48mol/L potassium chloride solution to pretreat FAU molecular sieve at 70°C for 22 hours, washing, drying at 80°C for 4 hours, and calcining at 400°C for 2 hours to obtain modified molecular sieve;

2) Step 1) modified molecular sieve is added to deionized water to form a suspension, the dilute aqueous ammonia solution of 4.8 mmol/L is used to dissolve potassium chloropalladate and palladium chloride to obtain a mixed solution, and the mixed solution is added dropwise to the suspension. , the mass ratio of palladium element and modified molecular sieve was controlled to be 0.024:1, the pH was adjusted to 8.0 with aqueous hydrochloric acid solution, the reaction was stirred at 92 °C for 46 hours, filtered and washed, dried at 90 °C for 1.5 hours, and then The muffle furnace was used to raise the temperature to 240 °C at a heating rate of 1.8 °C/min, and kept in the muffle furnace for 3.8 hours to obtain Pd-loaded molecular sieves;

3) Put the Pd-loaded molecular sieve obtained in step 2) into a mold, shape the catalyst at 80° C. and 950 MPa, and finally obtain a modified molecular sieve-loaded Pd catalyst.

FIG. 7 is a SEM image of the modified molecular sieve supported Pd catalyst prepared in this example, and it can be seen that the average particle size of the molecular sieve is 0.5 microns. The catalyst had a pore volume of 0.35 cm ³ /g and a specific surface area of 940 m ² /g as measured by nitrogen adsorption and desorption. Fig. 8 is the XRD pattern of the modified molecular sieve-supported Pd catalyst prepared in the present embodiment. It can be seen from the figure that there is no diffraction peak of metal palladium, indicating that Pd has a high degree of dispersion. The mass content of Pd was 2.4% as measured by inductively coupled plasma emission spectrometer. FIG. 9 is a TEM image of the modified molecular sieve-supported Pd catalyst prepared in this example after 12 hours of catalytic reaction, and it can be seen that the average particle size of the Pd particles is 13.1 nanometers.

Example 4

1) using 0.05mol/L ammonium fluoride solution to pretreat the EMT molecular sieve at 92°C for 22 hours, after washing, drying at 80°C for 4 hours, and calcining at 550°C for 2 hours to obtain the modified molecular sieve;

2) adding step 1) modified molecular sieves into deionized water to form a suspension, dissolving palladium nitrate with a 3.5mmol/L dilute aqueous ammonia solution to obtain a mixed solution, adding the mixed solution dropwise to the suspension, controlling palladium and The mass ratio of the modified molecular sieve was 0.015:1, the pH was adjusted to 9.0 with an aqueous ammonia solution, and the reaction was stirred at 80 °C for 36 hours. After filtration and washing, it was dried at 80 °C for 6 hours. The heating rate of °C/min was increased to 190 °C and kept in a muffle furnace for 2.5 hours to obtain a Pd-loaded molecular sieve;

3) Put the Pd-loaded molecular sieve obtained in step 2) into a mold, shape the catalyst at 50° C. and 400 MPa, and finally obtain a modified molecular sieve-loaded Pd catalyst.

Figure 10 is a SEM image of the modified molecular sieve supported Pd catalyst prepared in this example, and it can be seen that the average particle size of the molecular sieve is 4 microns. The pore volume of the catalyst was 0.33 cm ³ /g and the specific surface area was 913 m ² /g as measured by nitrogen adsorption and desorption in FIG. 11 . FIG. 12 is the XRD pattern of the modified molecular sieve supported Pd catalyst prepared in this example. It can be seen from the figure that there is no diffraction peak of metal palladium, indicating that Pd has a high degree of dispersion. The mass content of Pd was 1.5% as measured by inductively coupled plasma emission spectrometer. Figure 13 is a TEM image of the modified molecular sieve-supported Pd catalyst prepared in this example after 12 hours of catalytic reaction. From the TEM image, it can be seen that the hexagonal lattice of EMT molecular sieve is obvious, and the Pd particles are evenly embedded in the molecular sieve crystal. In the lattice, the average particle size of the Pd particles is 0.8 nm.

Comparative Example 1

1) with step 1) of embodiment 2;

2) with the step 2 of embodiment 2);

3) Put the Pd-loaded molecular sieve obtained in step 2) into a mold, shape the catalyst at 40° C. and 1200 MPa, and finally obtain a modified molecular sieve-loaded Pd catalyst.

FIG. 14 is the XRD pattern of the modified molecular sieve supported Pd catalyst prepared in Comparative Example 1. It can be seen from the figure that there are obvious diffraction peaks of metallic palladium (Pd ⁰ ), indicating that the degree of dispersion of Pd is low. The mass content of Pd was 1.1% as measured by inductively coupled plasma emission spectrometer. Figure 15 is a TEM image of the modified molecular sieve-supported Pd catalyst prepared in Comparative Example 1 after 12 hours of catalytic reaction. From the TEM image, large Pd particles with an average particle size of 100 nanometers (marked by white circles in the figure) can be observed, It shows that the sintering of metal Pd occurs.

It can be seen from Table 1 that the modified molecular sieve-supported Pd catalysts prepared in Examples 1 to 4 do not contain chlorine element, indicating that these catalysts are chlorine-free. The relative crystallinity of the molecular sieve represents the number of defect sites in the molecular sieve. The relative crystallinity of 100% indicates that the molecular sieve is basically free of defects, and the relative crystallinity of 0% indicates that the molecular sieve is all defect sites. At this time, the structure of the molecular sieve is completely destroyed. It can be seen from Table 1 that when the molding temperature and molding pressure are too low, there are fewer molecular sieve defect sites, and Pd is easy to form larger Pd particles (Example 1); when the molding temperature and molding pressure are too high, there are many molecular sieve defect sites. , the molecular sieve is severely broken, and Pd is also easy to form larger Pd particles (Example 3); when the molding temperature and molding pressure are within a reasonable range, the molecular sieve defect position is moderate, and the Pd particle size at this time is small (Example 2 and Example 4).

Only changing the molding pressure of step 3) in Example 2, from 350 MPa in Example 2 to 1200 MPa in Comparative Example 1, it can be seen that the average particle size of Pd particles increases from 0.3 nm to 100 nm , which further proves that the forming pressure can significantly change the particle size of metallic Pd.

The following conclusions can be drawn from Table 1. Changing the catalyst molding conditions can effectively adjust the size of the Pd particles.

Table 1 Physical and chemical properties of modified molecular sieve supported Pd catalysts prepared in Examples 1 to 4 and Comparative Example 1

Application example

The modified molecular sieve supported Pd catalysts prepared in the above-mentioned Examples 1 to 4 and Comparative Example 1 were evaluated for catalytic activity on a continuous-flow fixed-bed reactor. The tubular reactor was 36 cm in length, 8 mm in inner diameter, and the catalyst loading was 0.1 g. The reaction takes carbon monoxide and methyl nitrite as raw material gas, nitrogen as diluent gas, and the ratio of gas flow rate is carbon monoxide: methyl nitrite: nitrogen (CO: CH ₃ ONO: N ₂ )=1:6:33, in the reaction raw material Does not contain any chlorine element, the reaction temperature is 110 ℃, and the reaction pressure is low pressure. The obtained product is directly analyzed by on-line gas chromatography, and the products include the main product dimethyl carbonate (DMC), the by-products dimethyl oxalate (DMO), methyl formate (MF) and dimethoxymethane (DMM). From this, the conversion of carbon monoxide X _CO , the selectivity of dimethyl carbonate based on methyl nitrite S _DMC/MN and the selectivity of each by-product based on methyl nitrite S _DMO/MN , S _MF/MN and S were calculated _DMM/MN .

It can be seen from Table 2 that the CO conversion and DMC selectivity of the modified molecular sieve supported Pd catalysts prepared in Examples 1 to 4 are significantly higher than the catalytic performance of Comparative Examples 1 to 3, which indicates that the modified molecular sieve supported by the present application is supported. Pd catalysts have relatively high selectivity and conversion.

Table 2 Catalytic performance of catalysts of Examples 1 to 4 and Comparative Examples

In Comparative Example 2 of the prior art, after 150 hours, the CO conversion was reduced to 75% of the initial CO conversion, indicating that the catalyst had poor stability. After the stability evaluation, it was found that Examples 1 to 4 could run stably for more than 300 hours, and the CO conversion and DMC selectivity remained basically unchanged. Among them, Figure 16 shows the stability of the modified molecular sieve supported Pd catalyst prepared in Example 2. test chart. After 300 hours of reaction, it was found by TEM characterization that the sizes of Pd particles in Examples 1 to 4 were 6.5 nm, 0.5 nm, 13.5 nm and 1.2 nm, which were basically the same as the sizes of Pd particles in Table 1. It can be seen that the modified molecular sieve supported Pd catalyst prepared in the present application has relatively high stability and good anti-sintering performance.

The following conclusions can be drawn from Table 1 and Table 2: The modified molecular sieve supported Pd catalyst prepared in this application is a kind of high stability, high selectivity, high conversion, anti-sintering, chlorine-free, and adjustable Pd nanoparticle size. The catalyst, in particular, provides a high-performance catalyst for the reaction of carbon monoxide and methyl nitrite in the low-pressure gas phase synthesis of dimethyl carbonate.

Claims (6)

1. a preparation method of modified molecular sieve supported Pd catalyst, is characterized in that, utilizes catalyst molding condition to change the quantity of molecular sieve defect position, thereby changes the size of Pd particle, comprises the following steps: 1) at a temperature of 20-95 °C, the molecular sieve carrier is treated with a modified solution, filtered, washed, dried, and calcined at a temperature of 150-550 °C for 2 hours to obtain a modified molecular sieve, and the modified solution is hydroxide. Sodium, Sodium Chloride, Sodium Carbonate, Sodium Bicarbonate, Sodium Nitrate, Sodium Acetate, Potassium Chloride, Potassium Carbonate, Potassium Bicarbonate, Potassium Nitrate, Potassium Acetate, Acetic Acid, Oxalic Acid, Succinic Acid, Citric Acid, Ethylene Diamine One or more combinations of tetraacetic acid, hydrofluoric acid and ammonium fluoride, the total concentration is 0.01～0.5mol/L, and the treatment time is 0.5～24 hours, and the molecular sieve is a kind of FAU molecular sieve and EMT molecular sieve or a combination of both; 2) adding the modified molecular sieve of step 1) into the aqueous solution and stirring to form a suspension, dissolving the palladium precursor with a dilute aqueous ammonia solution of 0.2-5.0 mmol/L to obtain a mixed solution, adding the mixed solution to the suspension, and controlling The mass ratio of palladium element and modified molecular sieve is 0.001～0.025:1. According to the pH value of the mixed solution, use inorganic acid or inorganic base to adjust the pH of the solution to 5～10, and stir the reaction at 5～95℃ for 0.5～48 After filtration, washing, drying at 5-95°C for 1-48 hours, and then roasting in a muffle furnace to obtain Pd-loaded molecular sieves; 3) Put the Pd-loaded molecular sieve obtained in step 2) into a mold for molding, change the number of molecular sieve defect sites through catalyst molding conditions, thereby changing the size of the Pd particles, and finally obtain a modified molecular sieve-loaded Pd catalyst, the catalyst The molding temperature is 40 ~ 50 ℃, and the molding pressure is 350 ~ 400 MPa. 2. the preparation method of a kind of modified molecular sieve supported Pd catalyst as claimed in claim 1, is characterized in that, in described step 2), palladium precursor is palladium nitrate, palladium acetate, palladium chloride, ammonium chloropalladate , one or more combinations of potassium chloropalladate, tetraammine palladium chloride, tetraammine palladium nitrate; described inorganic acid includes one or more combinations of hydrochloric acid, nitric acid; described inorganic base Including one or a combination of ammonia water, sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, potassium carbonate and potassium bicarbonate. 3. the preparation method of a kind of modified molecular sieve supported Pd catalyst as claimed in claim 1, is characterized in that, in described step 2), the heating rate of muffle furnace is 0.2～2.0 ℃/min, and the roasting temperature is 110 ℃ ~250°C, calcination time is 0.5 to 4 hours. 4. the preparation method of a kind of modified molecular sieve supported Pd catalyst as claimed in claim 1, is characterized in that, the active component of the modified molecular sieve supported Pd catalyst that step 3) obtains is zero-valent Pd and valence Pd, carrier In order to modify the molecular sieve, the mass fraction of Pd in the catalyst is 0.1% to 2.5%, and the average particle size of the Pd particles is 0.2 to 15 nanometers. 5. the preparation method of a kind of modified molecular sieve supported Pd catalyst as claimed in claim 1 is characterized in that, in the modified molecular sieve supported Pd catalyst obtained in step 3), the average particle size of molecular sieve is 0.1～4 microns, and the pore The volume is 0.21-0.37 cm ³ /g, and the specific surface area is 800-950 m ² /g. 6. the application of the catalyst obtained by the preparation method of a kind of modified molecular sieve supported Pd catalyst as claimed in claim 1 in gas-phase synthesis of dimethyl carbonate, it is characterized in that, described modified molecular sieve supported Pd catalyst application Synthesis of dimethyl carbonate from carbon monoxide and methyl nitrite by low pressure gas phase method.

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